Currently, in the field of security systems, motion detectors are often provided to detect intruders. Many motion detectors incorporate passive infrared (PIR) technology and/or microwave (MW) technology.
PIR technology has long been used in motion detectors. The PIR sensor detects the difference between the infrared energy emitted from an intruder and that emitted from the ambient environment. Many PIR detectors utilize Fresnel lenses or custom shaped mirrors to focus infrared energy on a pyrodetector. The output signal from the pyrodetector is then processed via analog hardware and/or digital signal processing. Lenses and mirrors are designed to provide various detection zones emanating radially from the sensor. As a target moves across the PIR detection zones, the sensing elements within the pyrodetector are alternately exposed to the target IF energy, resulting in an alternating voltage output from the PIR sensor. The amplitude and frequency of this voltage vary with a number of factors including target size, speed, and direction relative to the PIR zones, difference between ambient and target temperature, width and spacing between the detection zones, and frequency response of the pyrodetector.
Upon receiving the signals, the detector may perform processing by comparing the received signal to one or more voltage thresholds. These threshold crossings produce positive and negative pulses that can be counted and timed, with certain combinations of pulse height, duration, and frequency being considered PIR alarms.
MW technology often operates on the principle of phase shift or Doppler effect. Unlike PIR, MW technology is an active technology. The MW detector transmits MW energy, which reflects off objects and returns to the MW detector. Moving objects result in a received signal that is frequency shifted from the original transmitted signal. The detector receives this signal, and generates an alternating voltage difference frequency signal which is then processed via hardware or digital signal processing. Processing may include comparison of the MW signal to one or more thresholds with certain combinations of quantity, duration, or frequency of threshold crossings considered MW alarms.
Intruders may attempt to sabotage or tamper with the motion detection components through various techniques. For example, intruders may attempt to mask detectors by coating a lens with an opaque substance (such as paint, tape or other object) that acts as a barrier between a motion detection sensor and the corresponding monitored space. Alternatively, intruders may attempt to cover or block the entire motion detector with an object or otherwise tamper with the motion detection components. Accordingly, security systems having motion detection components are often equipped with an anti-masking system that detects tampering with the motion detection components.
Anti-masking systems are typically designed to detect when a person attempts to cover or mask a motion sensor so that it cannot detect motion. The anti-masking function is typically performed by emitting an IR signal from the motion detector and detecting a reflection from a blocking object. Typically, a portion of the IR energy is directed through the lens of the detector to determine if something such as tape, spray paint or other article has been used to block the lens.
Intruders have developed certain techniques to defeat anti-masking functions in motion detectors. An illustrated embodiment of the anti-masking system and method of the present disclosure uses a plurality of different anti-masking functions executed at different times to reduce the likelihood that an intruder may be able to defeat the anti-masking function.
In an illustrated embodiment of the present disclosure, an anti-masking system is provided for a motion detector including a housing having a lens. The anti-masking system comprises at least one energy source, a spreading lens configured to receive energy from an energy source and to emit the energy outside the housing of the motion detector, a spreading lens sensor located inside the housing to detect energy emitted from the spreading lens which is reflected back into the housing through the lens from an object located outside the housing, and at least one reflector located outside the housing adjacent the lens. The at least one reflector is configured to reflect energy received from an energy source back into the housing through the lens. The anti-masking system also comprises a reflector sensor located within the housing, the reflector sensor detecting reflected energy from the at least one reflector to determine whether an object is located between the at least one reflector and the lens, and a retroreflector located on the housing proximate to the lens. The retroreflector is configured to receive and reflect energy from an energy source. The anti-masking system further comprises a retroreflector sensor located within the housing to detect energy reflected back into the housing by the retroreflector to determine whether an object is located on the retroreflector, and a controller configured to selectively supply energy from the at least one energy source to the spreading lens, the at least one reflector, and the retroreflector. The controller is configured to monitor signals from the spreading lens sensor, the reflector sensor, and the retroreflector sensor to determine whether the lens of the motion detector has been masked by an object.
In one illustrated embodiment, the controller is configured to control a timing circuit sequentially to supply energy from the at least one energy source to the spreading lens, to detect a response from the spreading lens sensor to determine whether an object is reflecting energy back into the housing through the lens, to supply energy from the at least one energy source to the at least one reflector, to detect a response of the reflector sensor to determine whether an object is located between the at least one reflector and the lens, to supply energy from the at least one energy source to the retroreflector, to detect a response from the retroreflector sensor to determine whether an object is located on the retroreflector, to start an anti-mask alarm timer in response to any detecting such objects, and to issue an anti-mask alarm when the anti-mask timer exceeds a predetermined trigger time. Performing these operations sequentially at separate times may reduce the likelihood that an intruder may defeat the anti-masking system. In another illustrated embodiment, the controller may cause the steps to be performed simultaneously.
In another illustrated embodiment of the present disclosure, a method is provided for controlling operation of an anti-masking system of a motion detector having a lens and a housing. The method comprises providing energy to a spreading lens to emit energy to an area outside the housing, monitoring a spreading lens sensor to detect an object reflecting energy emitted from the spreading lens back into the housing through the lens, providing energy to a reflector located outside the housing adjacent the lens, the reflector reflecting the energy back into the housing through the lens, and monitoring a reflector sensor to detect a decrease in an energy level received by the reflector sensor indicating that an object is located between the reflector and the lens. The method also comprises providing energy to a retroreflector located on the housing proximate to the lens, monitoring a retroreflector sensor to detect a decrease in energy reflected by the retroreflector back into the housing due to an object being located on the retroreflector, and issuing an anti-masking alarm in response to a detection of an object during the monitoring steps.
In one illustrated embodiment, the providing and monitoring steps are performed sequentially in order to reduce the likelihood that an intruder may defeat the anti-masking system. In another illustrated embodiment, the providing and monitoring steps may be performed simultaneously.
In yet another illustrated embodiment of the present disclosure, an anti-masking system is provided for a motion detector including a housing having a lens. The anti-masking system comprises an energy source, and a retroreflector located on the housing proximate to the lens. The retroreflector is configured to receive and reflect energy from the energy source. The system also comprises a retroreflector sensor located within the housing to detect energy from the energy source that is reflected back into the housing by the retroreflector, and a controller configured to selectively supply energy from the energy source to the retroreflector. The controller is also configured to monitor signals from the retroreflector sensor to determine whether an object is located on the retroreflector.
Additional features of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of illustrative embodiments exemplifying the best mode of carrying out the invention as presently perceived.